A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
It is desirable to project a pattern from the patterning device onto the substrate with a high accuracy. The accuracy with which the pattern is projected onto the substrate depends in part upon the optical wavefront of the radiation used to project the pattern. A deviation of the optical wavefront from its ideal shape may be characterized by one or more wavefront aberrations. A wavefront aberration can have a significant effect upon the accuracy with which a pattern is projected by the lithographic apparatus.
There are various sources of wavefront aberration in a lithographic apparatus, including for example non-ideal optical element (e.g., lens) fabrication, drift of an optical element property over time, and optical element heating. Since a wavefront aberration will often be present in a lithographic apparatus it is desirable to be able to reduce one or more wavefront aberrations in a controlled manner, thereby allowing the accuracy with which a pattern is projected by the lithographic apparatus to be increased.